KWU Steam Turbine Gov. & Protection System

Chapter 11

GOVERNING & PROTECTION SYSTEM 11.1

Features of KWU Turbine Governing

The increasing size of thermal units, plant complexity and use of high steam parameters demand sophisticated turbine control and instrumentation. The turbine control system developed for KWU Steam Turbine meets the demand of precise control with high response and safety. This sate-of-the-art control system has following functional areas. a) Electro-hydraulic governing as the main control system b) Hydraulic governing as the back-up control system c) Electrical turbine protection system with hydraulic operated turbine trip gear. The main control system, i.e. electro-hydraulic governing system, facilitates the operation of the turbo-set in an interconnected grid system. The electrical measuring and processing of signals, after the advantages such as flexibility, dynamic stability and simple representation of complicated functional relationship. The salient features are listed below:  The types of governor used are electro-hydraulic backed-up by hydraulic speed governor.  Throttle governing method is adopted to control the turbine load  Hydraulic governor always tracks the EHC during normal operation through tracking device  Electro-hydraulic control system has three control loops:  Speed control loop  Load control loop  Pressure control loop The signals for speed, load and M.S. Pressure are acquired by electrical transducers, processed in electronic circuit cards and then converted in to hydraulic pressure signal in electro-hydraulic converter. Hydraulic signal is further amplified for actuation of control valves. Speed, load and pressure control loops provide flexible operation of turbine in various modes of operation & are designed to encounter emergencies.  Regulation range of electro-hydraulic speed controller can be adjusted from 2.5 – 8%, even while the machine is in operation, in steps of 0.5%. The normal regulation setting is 5%.  Precise load frequency droop (in load controller) with high sensitivity.  Hydraulic speed governor regulation is set at 5%  Control valves open in proportion to signal of secondary oil pressure and actuated by constant control oil pressure.  Transient speed rise (TSR), i.e. maximum speed rise above rated speed when the turbine trips at full load: i) When control valves are mounted near turbine casing : 8% ii) When control valves are mounted away from the turbine casing : 8.5%  Oil required for governing system

: 100 lit/min.

 Dead Band (No response speed in %) : i) Electro-hydraulic governor ii) Hydraulic governor

: 0.01% : 0.1%

 Closing time of valves i) Emergency stop valve (ESV) ii) H P Control Valve (HPCV)

: 0.26 sec. : 0.4 sec.

Chapter-11: Governing & Protection System

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iii) Interceptor Valve (IV) iv) IP control valve (IPCV)

: 0.8 sec. : 0.8 sec.

 All governing components such as Main trip valves, EHC, follow-up pistons, starting and load limiting device, hydraulic governor are installed outside the turbine pedestal and assembled in hydraulic governing rack.  All types of turbine starts-ups are performed through HP control valves and they control the load from 0-100%. However, IP control valves have a controlling function alongwith HPCV upto 20% load. Above 20% load IP control valves remain open 100% and load control is performed by HP control valves 1 & 2.  The Trimming Device provided for the IP control valves, is used for controlling the HP exhaust pressure to prevent churning in HP turbine in case following conditions occur.  

2

HP exhaust pressure > 32 kg/cm and Load less than 20% of rated load

 The solenoid valves fitted on IP secondary oil and auxiliary secondary oil lines operate through energisation of load shedding (rejection) relay if following conditions occur.  Load throw off > 50% &  Balance load < 20% &  Grid frequency > 49 Hz When solenoid valve opens, it drain IP secondary oil and auxiliary secondary oil & de-pressurizes it resulting in to closure of HP & IP control valves for 1.3 sec. This prevents over speeding of turbine. Afterwards governing system controls the turbine load with proportionate opening of HP & IP control valves. One contact of load shedding relay operate the fast opening device of HP bypass valves to open it quickly to prevent the abrupt rise of pressure in M.S. lines.  Automatic turbine tester (ATT) is provided to check the operation of protective devices and normal closing of ESV, IV, HPCV & IPCV while the machine is running on load.  Automatic Turbine Run-up System (ATRS) is integrated with electro-hydraulic controller for automatic turbine start-up upto block load.  Isolated grid operation facility with reliable operation of turbine at block load.  Turbine Stress Evaluator/Controller (TSE, TSC) influence to prevent overstressing of turbine parts, is incorporated in speed controller and load controller of E.H.G. This ensures safe operation at all loads and steam conditions.  Interfacing of Coordinated Master Control with electro-hydraulic governing system and boiler master fuel controller for controlling the unit with single set point, i.e. unit target load set point.

11.2

Description of Governing System

The turbine is equipped with electro-hydraulic governing backed up by hydraulic governing. The governing scheme is designed to operate the turbine in integrated grid system to ensure stable and reliable operation in all eventualities. Fig. 11.1 shows the structure of turbine control system, comprising of electro-hydraulic controller and mechanical hydraulic controller. In case of EHC, the processed governing signal in the electrical voltage form is converted into hydraulic signal, which is further amplified to derive secondary oil pressures for operation of control valves. Similarly, the hydraulic speed signal (primary oil pressure) or the speeder

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Steam Turbines for Power Generation

Fig.11.1 Structure of Turbine Control System

Fig11.2 Action of Protection & Governing System on Turbine Stop & Control Valves


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gear position signal is converted into auxiliary secondary oil pressure and finally amplified to secondary oil pressure signal in mechanical hydraulic controller. Since the EHC & HC are connected in parallel to respective secondary oil lines (HP secondary oil and IP secondary oil), the minimum value secondary oil pressure signal passed on to the control valve servomotors to control the process. This way the controller with lower position takes over the other controller, which remains as back-up. The load of the turbine is controlled by throttling the main steam pressure upto full load range by HP control valves (2 Nos.) and by throttling the hot reheat pressure upto about 20% load range after 2 which IP control valves remain full open. The secondary oil pressure signal varying from 2.5 – 5 kg/cm opens the control valves from 0 – 100% in both the cases. 2

The protection oil circuit which generates trip oil pressure (6- 7.5 kg/cm ) acts on the servomotors of Emergency stop valves and Interceptor valves to open it fully. Hence in the absence of trip oil pressure, stop valves close rapidly due to spring force. The protection devices drain the trip oil in the event of electrical or mechanical tripping signal from protection logic. Fig. 11.2 shows the action of governing and protection system on turbine control stop valves & Fig.11.5 elaborates the governing & protection oil circuit.

11.3

Trip Oil Circuit

The trip oil circuit is explained in Fig. 11.3. The filtered control oil from AOP/MOP is supplied to remote trip solenoid valves 1 &2, connected in series, which generates trip oil. When the solenoids are de-energised, trip oil is passed to the main trip valves. In the event of electrical tripping signal from protection logic, solenoids are energised and in that case, control oil supply is blocked and trip oil is connected to drain for tripping of turbine.

Fig.11.3 Generation of Trip Oil

In the normal course, trip oil is passed through the main trip valves connected in series. When main trip valves are in lifted condition, trip oil is supplied at normal pressure (6 – 7.5 kg/cm2) through changeover valve to turbine stop valves (ESV & IV) and governing system for generation of secondary oil to regulate control valves position. The main trip valves are held in lifted position in turbine running condition by the auxiliary trip oil pressure. The auxiliary trip oil is connected to hydraulic protective devices viz. overspeed trip, thrust bearing trip and low vacuum trip.

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In the event of actuation of these hydraulic protective devices due to abnormal turbine condition, the auxiliary trip oil is drained rapidly and due to which main trip valves come to downward position. In this position, trip oil circuit is connected to drain and the trip oil pressure falls to zero. Due to absence of trip oil pressure turbine stop valves close rapidly and also the control valves due to falling of secondary oil pressure. This way turbine is tripped instantaneously. The changeover valve is employed in the trip oil circuit (Fig. 11.3) to change over the trip oil supply from protection circuit to test oil supply from solenoid valves during the automatic turbine tester (ATT) procedure. This permits the testing of the operation of protective devices without tripping the turbine on load.

11.4

Secondary Oil Circuit

The source oil for generation of secondary oil is trip oil. Secondary oil pressure decides the control valves position, which is in turn regulated by governing system. As shown in Fig. 11.4, trip oil from trip oil header is supplied to follow-up pistons of hydraulic converter and electro-hydraulic converter through orifices. The sleeves of follow-up pistons are operated by respective hydraulic governor and electro-hydraulic converter so as to adjust the trip oil draining rate through the ports. The resulting pressure is the secondary oil pressure which serves as signal for proportionate opening of control valves. Since the secondary oil generated by electro-hydraulic converter and hydraulic converter are connected in parallel to respective HP Secondary oil and IP secondary oil circuits, the governor with minimum position controls the process by generating required secondary oil pressure, i.e. the hydraulic minimum is achieved.

Fig.11.4 Generation of Secondary Oil

Electro-hydraulic Governing: The electro-hydraulic converter receives the governing signal in the form of electrical voltage from electrical governor. Electrical governor has three control loops as follows : 

Speed Control : Used for rolling the turbine upto rated speed and loading to block load


227

228

Steam Turbines for Power Generation Fig.11.5 Governing & Protection Oil System for KWU Steam Turbine



Load Control : For loading the turbine from block load to full load and normal operation



Pressure Control : This comes into service if the main steam pressure drops by limit set (generally 2 10 kg/cm ) with respect to pressure set point. This controller takes over the load controller automatically if above condition occurs so as to restore the main steam pressure by adequate closing of control valves. The selection circuit employed in the electric governor logic automatically selects the appropriate controller depending upon turbine control requirement. The output from selection circuit is passed to the valve position controller which checks the error between actual valve position and control signal and accordingly generates the output, which is finally supplied to electro-hydraulic converter. Electro-hydraulic converter converts the electrical signal into HP & IP secondary oil pressure signals for the actuation of control valves to control the turbine speed/load.

Hydraulic Governing: The hydraulic governor receives turbine speed signal in the form of primary oil pressure and reference speeder gear position. Accordingly the equilibrium is achieved and it generates auxiliary secondary oil pressure. Auxiliary secondary oil pressure signal operates the follow-up pistons of hydraulic converter, which in turn generate HP & IP secondary oil pressure signals. In normal course of turbine operation, speeder gear of hydraulic governor is positioned at 100% so that E.H.C. can assume the turbine control throughout the loading range since it remains at minimum position. The tracking device, which constantly adjust the position of starting and load limiting device with respect to EHC position ensures that turbine load does not rise abruptly in the event of failure of EHC (which goes to 100% in case of coil failure). S.L.L.D. locks the position of hydraulic governor bellow position for limiting the load.

11.5

Description of Control Loops

11.5.1 Speed Control Loop The block diagram of electro-hydraulic governing system is shown in Fig. 11.6, which represents three control loops viz. speed, load and pressure. The speed control loop is formed by a speed reference limiter, speed reference & speed controller. The speed controller compares the speed reference generated by the speed reference limiter circuit with the actual speed of the turbine and accordingly provides an output for valve position controller. The actual turbine speed (nact) is acquired by three digital speed pick-ups based on the hall probe principle. The output of each pick-up is processed in three different channels. The output from only one channel is used and other channel provides redundancy. The speed reference is generated by the speed reference limiter loop. The desired reference speed value is set with the help of potentiometer remotely from control desk or manually from the panel. In the speed reference limiter the reference speed signal is juxtaposed with the output from the turbine stress evaluator and a time dependent limited speed reference is generated corresponding to the highest permissible rate of speed increase, which is consistent with the safe operation of the turbine (max. rate 600 rpm/min, minimum rate 108 rpm/min). After attaining the target reference speed the output of the speed reference speed limiter is held constant. The output of speed reference limiter is automatically blocked in the event of a fault in T.S.E. and the speed of the set cannot be changed (speed reference blocked) until and unless TSE influence is made off.


229

230


Fig.11.6 Electro-Hydraulic Controller for KWU Steam Turbine

The speed controller exhibits a steady state proportional control to form the necessary droop characteristic. The dynamic response is improved by the addition of proportional integral and differential elements. The speed controller realizes the following operations: -

Start-up of the turbine Synchronization of the generator Provide a minimum load operation (10% of rated load).

11.5.2 Load Control Loop This comprises of following elements: - Load reference limiter - Frequency load drop - Load controller The reference load is set by a motor operated potentiometer, which is transmitted to the load reference limiter. The load reference set value and output signal from turbine stress evaluator are juxtaposed in the time dependent limited reference load setting depending upon the influence of the TSE at the highest permissible rate consistent with the thermal stresses of the turbine both during load increase and decrease. After attaining the target load reference, the reference limiter output is held constant. The turbine stress evaluator influence is rendered ineffective upon failure of TSE and the load reference limiter is held constant (blocked). It is possible to increase or decrease the load when turbine stress evaluator is disconnected. The load gradient setter also influences the time dependent load reference signal but the actual maximum permissible load rate is governed by the turbine stress evaluator. The grid controller can also be activated to exercise control on the load reference setter, which in turn is effective via the load reference limiter and TSE. This facilitates loading of the set from load dispatch centres. The load controller receives the reference load signal from the load reference limiter and also from the frequency controller depending on the frequency of the grid. These two signals are summed in the load controller and net value derived for controlling the valve position. The droop characteristics of the frequency controller can be varied from 2.5 to 8% in steps of 0.5%. To limit the total power delivered by the turbine, the system is equipped with maximum load reference limit, Pmax, which has priority over all other influences acting directly or indirectly on the valve position controller. The actual active load value is acquired in three independent channels and transmitted to the load controller. In case of a deviation of more than 5% in between the measurement channels, an alarm "actual load signal faulty" is initiated. The load controller exhibits a proportional-integral action and has an excellent dynamic response. The load controller comes into operation only when the turbine is synchronized and the block loading has been achieved. This controller and the speed controller signals are transmitted to valve position controller through maximum and minimum selection circuits. In case the sudden loss of export load, the output of the load controller is immediately reduced below the output of the speed controller, which is set at station load (10% of rated load). Due to maximum selection speed controller assumes control and returns the turbine fact to almost the rated speed. Hence this provision prevents the over speeding of the turbine. 11.5.3 Pressure Control Loop Operation of the initial steam pressure controller initiates the unloading of the set in case initial steam pressure falls below a preset value. The unloading of the set continues proportionally


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corresponding to available boiler pressure. The loading of the set begins only if the firing rate in the boiler is increased. On the other hand in the limit pressure mode, the pressure controller does not come into picture 2 unless the main steam pressure drops by preset limit (generally 10 kg/cm ) with respect to the M.S. pressure setpoint. In this case pressure controller takes over the load controller and closes the control valves to restore the pressure. During this period, load controller tracks the actual load value of generator and takes over the control as soon as pressure becomes normal and pressure controller output become more than the load controller output. The load controller/speed controller output and the pressure controller output are passed through the minimum gate. In normal course, pressure controller output is maximum so that load controller is always active. 11.5.4 Valve Position Controller The electrical voltage signal from the selection circuit (selected from speed, load or pressure control loop as per prevailing condition) is provided to valve position controller. This input signal is continuously compared to actual valve lift (feed back signal) in the valve lift controller and the error signal output is transmitted to the electro-hydraulic controller. The feed back signal representing the valve lift is derived from position of electro-hydraulic converter plunger as an analogue value from the differential transformer type transducer. There are two transducers (Collins) continuously scanning the position of the plunger. The characteristics of valve position (lift) controller are Proportional-Integral and differential type, which ensures high overall sensitivity and improves transient response. The outputs from the speed control loop and load control loop are juxtaposed and tied to each other by means of a maximum selection circuit. In a separate circuit the output of speed control loop is summed with load reference valve. This signal is compared with the signal from maximum selection circuit of speed and load controller and minimum of the two juxtaposed with the output signal from pressure control loop in yet another minimum selection circuit. The output of this minimum selection circuit is fed to valve position controller.

11.6

Starting and Load Limiting Device (SLLD)

The SLLD is clubbed with hydraulic governor. The functions of SLLD are :  To latch the protective devices and HP/IP stop valves (ESV, IV) when SLLD is brought to bottom most position (0%). In this position SLLD generates auxiliary start-up oil, which latches protective devices and also generates start-up oil to latch stop valves servomotor (ready for opening).  To open ESVs and IVs  To compress the governor bellows at the beginning of start-up so that control valves opening is prevented till ESVs and IVs are opened fully at 56% SLLD position. Further raising of SLLD causes control valves to open (when EHG is out of service) the speed of the turbine is raised to 2400 rpm (approx. 80% of rated speed). At this speed hydraulic governor takes over the control.  To provide the load limiting function in case of hydraulic governor operation or when EHC fails.

11.7

Load Rejection Relay (LRR)

The load rejection relay constitutes a supplement to the hydraulic controller to ensure that the speed of the turbine is kept safely below the response value of the mechanical-hydraulic overspeed trip device during bulk load trip device during bulk load rejection.

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The relay is mostly necessary when the hydraulic governor is controlling, but is also kept in operation when the EHC controls the turbo-generator. Upon actuation of the LRR, the HP & IP control valves are closed completely for a short interval (1.4 sec. max.), supporting the hydraulic control system. For activating the LRR following three conditions have to be fulfilled. i) ii) iii)

Load throw-off more than 50% Remaining load less than 20% of rated load Frequency more than 49 Hz.

The LRR measures the electrical load rejection at generator. Any load rejection of the turbo-set, i.e. a bulk negative load change, is therefore acquired instantaneously by the LRR and is then transformed into an electrical pulse, the duration of which is related to the magnitude of load rejection. This pulse energizes two solenoid valves:  One solenoid valve connects the auxiliary secondary oil (hydraulic output signal of hydraulic speed governor) supply line to a drain, causing a rapid drop of the pressure of that fluid and therefore closing of the HPCVs and IPCVs.  The second solenoid valve connects the secondary oil of IPCVs to a drain causing IP control valves to close rapidly than the action of first solenoid valve (due to its hydraulic characteristics). According to above arrangement, all the control valves close at a maximum speed. The fast response of the LRR is caused by measuring the change of generator load before the turbine speed rising starts. The LRR acts therefore earlier on the hydraulic control system, i.e. before the hydraulic control takes action due to speed increase. After the electrical pulse issued by the LRR has ended, the solenoid valves will close again. Auxiliary secondary and secondary oil pressures will build-up again, corresponding to balance load on the turbine since by this time hydraulic governor will resume normal control of the turbine. To prevent any response of LRR at frequencies below rated one, a frequency lock is employed. If the transmission system is operating below rated frequency while a load rejection occurs, without this lock-out, the LRR will issue its signal and cause the control valves to close immediately. After its signal has disappeared, however, the hydraulic speed governor will find a big mismatch between the actual speed and the hydraulic speed set point causing the turbine control valves to open rapidly and in turn accelerate the turbine that it most likely will run into an overspeed trip. Therefore, the frequency relay blocks the LRR signal below an adjustable value 49 Hz and releases it when this frequency is exceeded again. The contact of LRR is also provided to fast opening device of HP bypass system to open HP/LP bypass valves rapidly during load throw-off to prevent abrupt rise of steam pressures.

11.8

Trimming Device

In order to avoid excessive heating of HP exhaust during HP-LP bypass operation, a sequencetrimming device has been provided. This device comes in operation if HP exhaust pressure exceeds a 2 preset value (32 kg/cm ) and load drops below 20% of rated value. When the trimming device is operated, IP control valves are closed and do not tend to open unless HP control valves opened wide. Trimming device is operated through the solenoid-operated actuator provided to adjusting device for IP control valves, follow-up pistons.

11.9

Functioning of Protection System

The turbine protection logic comprises of protective devices function of which is to trip the turbine by instantly closing Emergency Stop and Interceptor valves thereby cutting off the steam supply. The obligatory turbine protection covers following functions.


233

Fig.11.7 Turbine Protection System

234


  

Protection of the turbo-set from inadmissible operating conditions. It prevents consequent damage in case of plant failure It restrict occurrence of failures to minimum,

The comprehensive turbine protection scheme is explained in Fig. 11.7. The design of the mechanical-hydraulic protection devices are in accordance with the hydraulic break-current principle and consists of :      

Two tripping devices, i.e. Main trip valves with integral manual trip arrangement Two remote trip solenoid valves operated by electrical protection system to cut-off trip oil supply Two channels of Electrical protection logic and turbine trip relays Two overspeed trip devices operated at 11% overspeed. One thrust bearing trip device (high axial shift due to thrust bearing wearing) One mechanical low vacuum trip device, which is a back up to electrical vacuum trip.

Operation: The operation of trip system close all stop and control valves of turbine rapidly. The main trip valves draw oil from pressure oil circuit cascaded through remote trip solenoid valves (ref. Fig.11.3). The oil coming out of main trip valves is supplied via changeover valve to ESV, IV, Secondary oil circuit and auxiliary secondary oil circuit of governing system as trip oil. The main trip valve remains in lifted condition (normal position) against the spring force by the auxiliary trip oil pressure acting below the spool. When the auxiliary trip oil pressure under the differential piston in the main trip valve falls below a certain adjustable value, due to the response of some protective device, the spring moves the piston downwards, opening the drain for the trip oil and closing the pressure oil inlet from remote trip solenoid valves. This in turn totally blocks oil to control circuits and causes closure of ESV, IV, Control Valves and extraction swing check valves. When the pressure in the trip oil circuit falls below a certain value, the upper piston and lower piston of the ESV & IV servomotors are separated by springs and valves are instantaneously closed. The turbine can be tripped manually by pressing the lever of the main trip valves or by pressing the "Emergency Trip" knob from the control room. The turbine protection system can be tested during operation. A device to initiate turbine trip during protective device testing is provided with the automatic turbine testing system so that the turbine is protected continuously during the period the protective devices are being tested.

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KWU Steam Turbine Gov. &amp; Protection System

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